It is the primary purpose of this project to study some of the pertinent factors which influence cell differentiation and malignant transformation, using techniques and approaches which range from the microscopic to the molecular level. Particular emphasis is given to those systems in which murine RNA tumor viruses or their precursors or chemical carcinogens may be the transforming agent. A variety of mouse model systems are used, including plasma cell tumors, mammary tumors, neuroblastomas, and methylcholanthrene-induced sarcomas. Current projects include: 1) effects of interferon on the assembly and maturation of murine retroviruses with special emphasis on the study of mechanisms whereby whole virions are rendered noninfectious; 2) effects of interferon on methylcholanthrene-induced sarcomas of the BALB/c mouse with the aim of defining anticellular and/or immunological activity; 3) relationship of retroviruses to the expression of certain phenotypic changes in chemical carcinogen-induced murine sarcomas.